Editable combobox overwrites my context menu - Fails

I have been trying to use the code in the reply to Question "Editable combobox overwrites my context menu" asked on asked Aug 30 '12 at 14:24 But it always errors with a nullreferenceerror on

(VisualTreeHelper.GetChild(VisualTreeHelper.GetChild(combobox, 0), 2) as TextBox).ContextMenu = combobox.ContextMenu;

In order to minimize any environment problems I have added the code as a new project

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://ift.tt/o66D3f"
        xmlns:x="http://ift.tt/mPTqtT"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <ComboBox IsEditable="True"    ContextMenuService.ShowOnDisabled="True" 
     Name="combobox" Loaded="combobox_Loaded">
            <ComboBox.ContextMenu>
                <ContextMenu>
                    <MenuItem Header="test"></MenuItem>
                </ContextMenu>
            </ComboBox.ContextMenu>
            <ComboBoxItem Content="Item1"></ComboBoxItem>
            <ComboBoxItem Content="Item2"></ComboBoxItem>
        </ComboBox>

    </Grid>
</Window>

code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        private void combobox_Loaded(object sender, RoutedEventArgs e)
        {
            (VisualTreeHelper.GetChild(VisualTreeHelper.GetChild(combobox, 0), 2) as TextBox).ContextMenu = combobox.ContextMenu;
        }  
    }
}